An obvious approach to the modification of emotional and motivational behavior would be through the control of the balance of brain catecholamines and indolamines, not by drug action, but more simply and directly by regulating the availability of their precursor amino acids. The work of others, notably Wurtman and his coworkers at MIT, indicates that it should be possible to do this by careful control of the dietary balance of the two essential amino acids, phenylalanine and tryptophan. The objective of the proposed research is to investigate the hypothesis that a dietary imbalance resulting in a phenylalanine excess favors arousal and aggression, and that imbalance favoring tryptophan opposes it. Male CF- 1 mice starting at 6 weeks of age will be maintained on semisynthetic diets of the following composition: 75% carbohydrate, 5% lipid, 2% vitamin mixture, 4% salt mixture, 2% cellulose, and 12% casein protein. Groups of 20 mice housed 4 per cage will be fed ad libitum the semisynthetic diets supplemented with L-phenylalanine, L-leucine, or L- tryptophan, each at 3 different levels in the range, 0.25 - 4.0%, of diet (replacing a small part of the dietary carbohydrate). Leucine, which is not metabolically related to either catecholamine or indolamine production, will be used as a control. Other control groups will be maintained on the unsupplemented semisynthetic diet or powdered Purina Rat-Mouse Chow. The weight of the animals determined weekly will be used as a measure of their general health and of the adequacy of the diets. After two months, behavioral consequences of the regimen will be determined using a highly reliable territorial aggression test devised two years ago by Thurmond and Quinton, and standardized since then with use of more than one thousand CF-1 male mice. The long range consequences of investigations of this type could provide a valuable adjunct to psychiatric treatment and the modification of human behavioral patterns.